A vehicle hydraulic brake system of the brake-by-wire type is known in which hydraulic pressure supplied from a high hydraulic pressure source is adjusted by a pressure regulator to a value corresponding to brake operation by the driver, or to a value set independently of brake operation by the driver, by an electronic control unit, and brake hydraulic pressure is generated by actuating a master piston of a master cylinder using the hydraulic pressure adjusted by the pressure regulator as assist pressure (see paragraphs [0062] to [0100] and FIGS. 3 and 4, of the below-identified Patent document 1).
A typical vehicle hydraulic brake system of this type includes an input piston (such as the input piston 413 shown in FIG. 3 of Patent document 1) configured to be advanced under the brake operating force, a pressure regulator (such as the hydraulic pressure control valve 434 shown in FIG. 3 of Patent document 1) which adjusts hydraulic pressure supplied from a high hydraulic pressure source to a value necessary to generate a target braking force and feed the thus adjusted hydraulic pressure into a third hydraulic pressure chamber (such as the rear hydraulic pressure chamber R3 shown in FIG. 3 of Patent document 1), and a master cylinder (such as the one shown in FIG. 3 of Patent document 1 at 411) configured to generate brake hydraulic pressure by driving a master piston (such as the intermediate piston 414 shown in FIG. 3 of Patent document 1) under the hydraulic pressure adjusted by the pressure regulator.
The input piston is axially slidably inserted in a hole formed in the rear end of the master piston such that the front end of the input piston protrudes into an inter-piston chamber (such as the one shown in FIG. 3 of Patent document 1 at R5) formed in the master piston. The rear end of the master piston protrudes into the third fluid chamber. The gap between the inter-piston chamber of the master piston and the third fluid chamber is sealed by a high-pressure seal (such as the seal member 463 shown in FIG. 3 of Patent document 1) disposed between the inner peripheral surface of the bore of the master piston and the outer periphery of the input piston.